Known tube mills can be used for grinding material such as ore. Large particulate material, such as gold ore, is delivered into the grinding pipe of the tube mill where it can be mixed with grinding media charges such as balls. Water can also be added to this mixture. It is not uncommon for the use of a tube mill to be intermittent, either due to an intermittent supply of material to be ground, due to maintenance of the mill itself or due to an emergency stop of the system. During the periods when the tube mill is not is use, the charge within the grinding pipe can consolidate and become firmly stuck to the inner wall of the grinding pipe, this is referred to as “frozen charge”. When the tube mill is set in motion again after a period of non-use during which frozen charge has occurred, there is a high likelihood that the frozen charge may become detached at the highest point of rotation of the grinding pipe. This will result in the frozen charge dropping onto the inner surface of the grinding pipe at the lowest point of rotation which, given the potential height of drop and the materials involved, could result in substantial damage occurring to the tube mill.
In view of this, known procedures called for a grinding pipe to be checked for frozen charge, and when detected, rotation of the tube mill is ceased immediately. Until recently the removal of frozen charge was a laborious manual process involving the use of air compressed hammers upon the charge which may, or may not, have first been softened by spraying with water.
Patent application publication DE 3528409 A1 describes an arrangement which detects the presence of frozen charges and stops rotation of the drum in the affirmative. If a tube mill is driven with constant angular speed, the corresponding torque increases to an absolute maximum indicative of loose charges starting to tumble towards the lower parts of the rotating tube. Under the presence of frozen charges however, such maximum is not observed at moderate angles of rotation.
Recently, a more efficient method and associated devices for removing such frozen charge has been disclosed in U.S. Patent Application No. 2008/0169368 (Becker et al.). This method involves controlling a gearless drive of a ring motor surrounding the grinding pipe to effect targeted detachment of frozen charge. The grinding pipe drive is operated to rotate the grinding pipe in an angular range and at an appropriated speed such that falling material does not cause damage to the grinding pipe or other components of the tube mill. An angle of rotation is set to oscillate about a predetermined angle of rotation, with a corresponding torque reference, or mean, value decreasing proportionally to the fraction of frozen charge.
A system with a driving torque applied to the grinding mill that can be both positive and negative is not suitable in mills having a geared drive as this can create a backlash of force on the gear teeth which in time will cause damage to the gears and will subsequently decrease the lifetime of the drive train.